Fireforming is done to change the shape of the case. A nice example. I have a couple of 9.3 x 57mm rifles. The only brass available is Norma, at about 4 bucks each. 8x57 is much more available, and lots cheaper. The only difference between the two is that one is 32 caliber and the other is 37. So you take an 8mm case, prime it, stick about 14 or 15 grains of bullseye in it, and put a piece of folded up toilet paper in the mouth. Chamber it in your 9.3 x 57 rifle, point it at the sky, and pull the trigger. It goes, "Pop", and swells (fireforms) to fill the chamber. Eject the shell. You now have a 9.3 x 57 case that says 8 x 57 on the headstamp. Continue doing as needed.

Somebody, down this board, is talking about making 8x57 out of 30/06. Works fine. Shove an ought six up an eight mil die, trim to length, and fireform, to blow the 30 caliber mouth out to 32 caliber.

Sometimes you have no other option. 30 and 35 Herret are hunting rounds that are made from 30/30 brass. To my knowledge, there is no factory ammo or brass made for them. So you cut your 30/30 to the right size, and fireform it to your chamber. Sometimes you do it to save money (my 9.3 example). And sometimes you do it just because you have bunch of brass that you otherwise have no use for (I make 7x57 out of 270 Winchester cases).

Sometimes people attempt to "improve" a cartridge. This generally involves moving the shoulder of the case forward and changing the angle to a sharper one. Look at a 22 Hornet http://stevespages.com/jpg/cd22hornet.jpg and compare it to a 22 Hornet, Improved http://stevespages.com/jpg/cd22hornetimproved.jpg . These two things will give you more powder capacity in the same size case. So, once you've changed your chamber size, you fire factory ammo in it, and the brass will fireform to the "improved" chamber.

Thanks for clearing that up for me. I thought it had something to do with heating up the case or something, but I get it now fire the gun to form a custom sized case thus fireforming. When using this process the case is form fit to your specific chamber so would this be something that competition shooters would probably do as well? Thanks again.

Just another example... My #4 enfield rifle is chambered for the .303 british cartridge, Like all military enfields, the chambers are 'out of spec' so to speak, to accomodate battlefield conditions. My enfield WILL NOT put 10 rounds inside of 4 inches at 100 yards with factory spec ammo. So, to remedy the problem, I simply necksize a fireformed case in order to preserve the demensions of MY chamber. The result is a rifle, that when loaded with these special .303 cartridges, will put all 10 rounds in the magazine into 2 inches or better at the same 100 yards. My enfield ammo will not chamber in another enfield of the same caliber. At least I have not found one that will. Below are some pics to show the difference in factory and my handloads.

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I fireform 375 win. from 30.30 cases although there is a slight difference in the head diameter I simply change the shell holders from no. 2 to no.3 and follow the procedure described by ALPO. Now something I don´t understand: Annealing. What are its benefits. And I have seen in some magazine that the cases are put on their bases in a container of water that covers them until some 1 or 1 1/2 inch, the heating the uncovered portions with a torch.
Could You please explain this with some detail? Thanl You

In addition to Alpo's responce above....fire forming is also done to have a case exactly match the rifle's chamber it was fired in....so that if you handload said brass you can improve accuracy becuase the reloaded round fits more exactly in the rifle's chamber.....so actually, every round fired from every rifle is fire formed to that rifle's particular chamber.
However, when you handload fireformed brass, if you full length resize the brass then it is no longer fireformed...but, if you neck size the brass only (or maybe bump the shoulder a little) and then load it up, the resulting round is said to be a better fit for that particular rifle....and thus better accuracy.

Just another example... My #4 enfield rifle is chambered for the .303 british cartridge, Like all military enfields, the chambers are 'out of spec' so to speak, to accomodate battlefield conditions. My enfield WILL NOT put 10 rounds inside of 4 inches at 100 yards with factory spec ammo. So, to remedy the problem, I simply necksize a fireformed case in order to preserve the demensions of MY chamber. The result is a rifle, that when loaded with these special .303 cartridges, will put all 10 rounds in the magazine into 2 inches or better at the same 100 yards. My enfield ammo will not chamber in another enfield of the same caliber. At least I have not found one that will. Below are some pics to show the difference in factory and my handloads.

Click to expand...

Wow. That's a big difference. I suspect I will be fireforming my Dad's 30-40 krag when I finally find ammo for it for the same reasons. I can't wait.

Here's a pic of a typical fire-forming comparison. The cartridge on the left is a factory loaded 275 Roberts. After firing this cartridge in a 257 Roberts Ackley improved chamber the taper is removed from the standard cartridge and the shoulder is moved forward and reformed to 40 degrees. The benefit is about a 20% increase in powder capacity and approx. 200-400 fps increase in velocity.

I have measured accuracy during fire-forming of this factory round and found it to be sufficient for most hunting conditions, so in a pinch you can always use factory ammo.

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The thinking and annealing techniques for wildcats were much different 25 years ago...maybe because 30% of the body needed to be annealed along with the neck/shoulder area as the body was also blown out towards the top...a whole 10 thousanths or more (in diameter)...as shown below...back then we used the molten lead technique for annealing...primarily for it's uniformity...quenching the just annealed neck area was considered counter productive and unnecessary uneven shock to the neck

This particular cartridge utilizes the Vom Hofe type double angle shoulder that lends itself to easier extraction when neck sizing only. The designer of this cartridge (1970's) was looking for .257 Weatherby ballistics out of surplus '06 cases using 1920's concepts left over from Newton Arms (a local company). Sadly he's gone...and now I anneal and load for it...